Process of treating stainless steels



@ct. 21, 1959 TATSUQ MA ET AL 3,473,973

PROCESS OF TREATING STAINLESS STEELS Filed May 11, 1966 FIG.

United States Patent PROCESS OF TREATING STAINLESS STEELS TatsuoMaekawa, Nobuo Nakajima, and Masaru Kagawa, Urawa, Saitama, Japan,assignors to Mitsubishi Atomic Power Industries, Inc., Chiyoda-ku,Tokyo, Japan Filed May 11, 1966, Ser. No. 549,780

Claims priority, application Japan, May 13, 1965, 40/27,630 Int Cl.(122E 1/02; C22c 39/26, 39/48 US. Cl. 14812.3 7 Claims ABSTRACT OF THEDISCLOSURE This invention relates in general to a process of treatingstainless steels and more particularly to a treating process ofimparting heat resistant property to stainless steels of the austeniticstructure.

Stainless steels of austenitic structure are generally poor in heatresistant property. At elevated temperatures they not only rapidlyincrease in corrosion rate and suddenly decrease in mechanical strengthbut also become brittle. This is primarily caused from the fact thatwhen the stainless steels of austenitic structure are heated at elevatedtemperatures for long intervals of time, the sigma phase is precipitatedat the grain boundaries therein resulting in a decrease inanti-corrosion and in an increase in brittleness. Therefore the existingstainless steels of austenitic structure are forced to be used atrelatively low temperatures.

Heretofore it is the prevailing conception that the abovementioneddecrease in both anti-corrosion and mechanical strength of theaustenitic stainless steels at elevated temperatures cannot beinherently avoided in view of their properties. For this reason, heatresistant nickel base alloys such as expensive Inconel-and Hasteloy havebeen commonly employed at elevated temperatures. It is very desirableand extremely economically advantageous to provide the type ofaustenitic stainless steels inexpensive and improved in anti-corrosionto allow them to be actually used at elevated temperatures.

Accordingly, it is an object of the invention to provide a new andimproved type of stainless steels increased in anti-corrosion andcapable of being actually used at higher temperatures than thosepreviously possible by in creasing the anti-corrosion of the austeniticstainless steels.

, With the above cited object in view, the invention resides in atreating process of imparting heat resistant property to stainlesssteels of austenitic structure having a composition including, byweight, from 6 to 22% of nickel, from 16 to 35% of chromium, up to 2% ofman- 3,473,973 Patented Oct. 21, 1969 ganese, up to 1.5% of silicon, upto 0.122% of carbon and the balance being iron except for very smallamounts of incident impurities which process comprises the steps of coldworking a body of said stainless steel to precipitate the ferritic ormartensitic phase therein, and heat treating the said cold worked bodyat a temperature of from 500 to 800 C. for a period of time sufiicientto convert the precipitated ferritic or martensitic phase into the sigmaphase.

The invention will become more readily apparent from the followingdetailed description taken in conjunction with the accompanying drawingin which:

FIGS. 1 and 2 are microphotognaphs of specimens cut from sheets ofstainless steels treated in accordance with the teachings of theinvention.

As previously stated, it is well known that if stainless steels ofaustenitic structure are heated at elevated temperature for long periodsof time then the sigma phase is precipitated at the grain boundariestherein with the result that they become brittle and decrease inanti-corrosion. Heretofore this deteriorative effect has prevailinglybeen considered to be incapable of being inherently avoided in view ofthe properties of the austenitic stainless steels.

It has now been found that the sigma phase itself has excellent heatresistant and anti-corrosive properties. It has also been discoveredthat uniform distribution of the fine sigma phase in 'austeniticstainless steel is not only effective for preventing embrittlement ofthe steel due to precipitation of the sigma phase at the grainboundaries but also makes it possible to greatly increase the heatresistant and anti-corrosive properties of the stainless steel.

In order to uniformly precipitate the fine sigma phase in a body ofaustenitic stainless steel in question, the invention contemplates firstto cold work the body of stainless steel to uniformly precipitate thefine ferritic phase or the fine martensitic phase or a mixture thereofand then to heat the cold worked body at a temperature of from 500 to800 C. for a period of time of from several to several hundred hours.This results in stainless steel having fine sigma phase grains in alarge amount uniformly distributed therein.

Austenitic stainless steels suitable for treatment according to theprocess of the invention may have any composition capable ofprecipitating the ferritic or martensitic phase therein by a coldworking operation. However, it has been found that for the best result,the composition of the austenitic stainless steel includes, by weight,from 6 to 22% of nickel, from 16 to 35% of chromium, up to 2% ofmanganese, up to 1.5 of silicon, up to 0.12% of carbon and the balancebeing iron except for very small amounts of incident impurities. Inorder to decrease the total amount of carbides which might beprecipitated by heat treatment, austenitic stainless steels low incontent of carbon may be preferably used. If desired, austeniticstainless steels suitable for' treatment according to the presentprocess may additionally include, by weight, up to 3% of molybdenum, upto 1% of niobium and up to 1% of titanium.

It has been found that the many types of cold working processes, thatis, cold swaging cold, rolling and cold reducing processes are performedto a reduction in area of from 10 to with good results. Cold working maybe performed to a magnitude of from 10 to 95% below room temperaturewith a satisfactory result. Also the austenitic stainless steels may besubject to a shot peening operation until the ferritic or martensiticphase appears in the surface layers thereof alone.

Since an amount of the sigma phase precipitated is approximatelyproportional to an amount of the ferritic or martensitic phaseprecipitated in the cold Working operation the composition of austeniticstainless steel and the degree of cold working can be appropriatelyselected to control the amount of ferritic or martensitic phaseprecipitated thereby to provide stainless steel including the desiredsigma phase. It has been found that the particle size and distributionof the precipitated sigma phase grains distribution of sigma phasegrains such as illustrated in FIG. 1 wherein a microphotograph magnifiedby a factor of 2000 is illustrated for such a specimen as Heat No.18-l0L depleted in the sigma phase precipitated after cold rolling andheat treatment. On the other hand four of the plus signs correspond tothe amount and distribution of sigma phase grains such as illustrated inFIG. 2 wherein a microphotograph illustrated for a specimen such as HeatNo. 19-9L enriched with the sigma phase precipitated after cold rollingand heat treatment. Two or three plus signs mean the amount anddistribution of sigma phase grains intermediate those illustrated inFIGS. 1 and 2 respectively. It has been found that the control sheetsincluded no sigma phase.

TABLE I.ANTI-CORROSION CHARACTERISTICS OF STAINLESS STEELS CorrosionWeight Chemical Composition in wt. percent Amount of Gain in mg./dm.

Sigma Phase 1,000 hours Inven- Conven- Inven- Conven- Heat No C Si Mn NiCr M Nb Ti tion tional tion tional The following examples illustrate thepractice of the 1 invention.

EXAMPLE I Sheets of austenitic stainless steels having differentcompositions listed in the following Table I were cold rolled to areduction of 80% at room temperature to transform a part of theiraustenitic structure to the ferritic or martensitic structure and thenheated in a heat treatment furnace at 600 C. for 25 hours. The heatedsheets were allowed to cool to room temperature within the furnace. Thesheets thus treated were exposed to hot, high pressure steam at 600 C.under a pressure of 100 kgs./ cm. for 1000 hours. Then the sheets weredetermined in terms of corrosion weight gain in mgs./dm. 1000 hrs. andthe results are listed in Table I.

Also a control series of the same austenitic stainless steels wereconducted with the same corrosion test after they were heat treated at1050 C. for /2 hour followed by water quenching in conventional mannerwithout cold working. The results also are listed in Table I. The amountand distribution of sigma phase grains precipitated in each of thetested specimens by the abovementioned cold rolling and heat treatingoperation is symbolically designated by the number of plus signs. Aminus sign indicates no sigma phase. A single plus sign corresponds tothe amount and From Table I, it will be appreciated that the austeniticstainless steels treated according to the present process are greatlyexcellent in corrosion resistance as compared with those annealed in theconventional manner. Table I indicates that the higher the amount of theprecipitated sigma phase the more the corrosion resistance will beimproved. Further it has been proved that the amount of precipitatedsigma phase depends upon the amount of the ferritic or martensiticstructure transformed from the austenitic structure during the coldrolling of the invention.

EXAMPLE II AISI Type 304L austenitic stainless steel including byweight, 0.013% of carbon, 0.27% of silicon, 1.07% of manganese, 9.6% ofnickel and 18.5% of chromium was made in rods having a diameter of 10mms. The rods were cold swaged to a reduction of 60% to transformationprecipitate several tens precent of the austenitic structure to theferritic or martensitic structure and then heated at 650 C. for 25 hoursto form several tens percent of the sigma phase therein. The heatedsheets were allowed to cool to room temperature. Control specimens wereheat treated at 1050 C. for /2 hour followed by water quenching in theconventional manner without cold working.

All the rods thus treated were exposed to hot, high pressure steam at25, 300, 400, 500, 600, 700 C. under a pressure of kgs./cm. for a longinterval of time (i.e., 1000 hours) to determine the corrosion resistantcharacteristics. Also the mechanical properties were measured by usingcircular test pieces in. in diameter with a gauge length of 1 in. Themeasurements were made between room temperature and 700 C. The resultsare listed in Table H.

TABLE II.MEGHANICAL AND CORROSION PROPERTIES OF STAINLESS STEELS AT ROOMTEMPERATURE AND ELEVATED TEMPERATURES Mechanical Properties Reduc- 0.2%tion Corrosion Test Yield Ultimate Elonin Weight Temper- StrengthTensile gatiou Area Gam in Specimen ature in Strength in in mgJdmfl/Treatments in C. kg lmtu. in kgJmm 3 percent percent 1,000 hrs.

Invention 25 50 87 40 40 300 48 56 27 60 i 400 45 51 28 60 9 500 35 4232 63 8 600 27 30 45 68 8 700 20 21 60 7 Conventional- 25 16 56 75 58300 8 38 40 7 3 400 7 37 38 70 11 500 7 32 35 66 48 600 6 28 33 48 464700 5 22 42 44 From Table II, it will be appreciated that the productstreated in accordance of the process of the invention are far high instrength and good in corrosion resistance as compared with theconventional annealed products. It is noted that the invention providesstainless steels having not only an extremely high 0.2% yield strengthbut also a high elongation, a high tenacity and excellent corrosionresistant properties at high temperature.

EXAMPLE III Rods identical to those used with Example II were firstcooled in liquid nitrogen and then cold swaged to a reduction of at -l96C. followed by heat treatment at 650 C. for hours. The resulting rodsexhibited substantially the same effects as rods cold swaged to areduction of more than followed by the similar heat treatment. Thismeans that cold Working at low temperatures is more effective fortransformation of the austenitic to the martensitie structure than aworking at room temperature. In other words, a lower temperature permitsa working degree to reduce with the same result.

EXAMPLE IV Pipes of an austenitic stainless steel substantially similarin composition to that used with Example 11 and having an outsidediameter 28 mms. and a wall thickness of 2 mms. were cold worked intopipes 19 mms. in outside diameter and 1 mm. in wall thickness at roomtemperature by a Cold Pilgar Process and then subject to a heattreatment at 650 C. for 25 hours. The results were substantially similarto those in Table II.

EXAMPLE V Steel sheets made of Heat No. 19l0L (see Table I) were subjectto shot peening to strongly work their surfaces and then to heat at 600C. for 25 hours. The sheets thus treated had corrosion resistancecharacteristics substantially similar to those shown in Table I.

The following Table III shows a comparison of AISI Type 3041. stainlesssteel treated in accordance with the present and conventional processesand known nickel alloys in terms of mechanical properties at 25 C. andcorrosion resistance to hot steam at 600 C. under a pressure of 100kgs./cm. Table III indicates that the stainless steel treated accordingto the invention is comparable to Inconel and Hasteloy in mechanicalstrength at room temperature and in corrosion resistance to hot steam.

TABLE III.MECHANICAL PROPERTIES AND CORROSION SPKNCE OF STAINLESS STEELSAND NICKEL Mechanical Properties at 25 C.

0.2% Ultimate Corrosion Yield Tensile Weight Strength Strength Elon-Gain in in in gation in ingldmfi/ Alloy kgJmln. kgJmmfl percent 1,000hrs.

Conventional AISI Type 304L Stainless Steel 2O 53 78 294 Present AISIType 304L Stainless From the foregoing, it will be appreciated that thepresent invention provides stainless steels greatly improved in heatresistance and anti-corrosion by the formation of the sigma phase grainsuniformly distributed in a large amount therein.

What we claim is:

1. A process for obtaining stainless steel highly corrosion resistanteven at elevated temperatures, from stainless steel of austeniticstructure having a composition by weight of 6 to 22% of nickel, from 16to 35% of chromium, up to 2% of manganese, up to 1.5% of silicon, up to0.12% of carbon, and the balance being iron except for very smallamounts of incidental impurities, which comprises cold Working at leastpartly a body of said stainless steel to transform at least part of theaustenitic structure to a structure selected from the group consistingof ferrite and martensite and then heating the cold worked body of steelfor a period of time at a sufiicient temperature to transform theresultant ferritic or martensitic structure into the sigma phase.

2. A process of treating a body of stainless steel of the austeniticstructure having a composition including, by weight, from 6 to 22% ofnickel, from 16 to 35% of chromium, up to 2% of manganese, up to 1.5% ofsilicon, up to 0.12% of carbon and the balance being iron except forvery small amounts of incident impurities, comprising the steps of coldworking at least partly the body of stainless steel to transform atleast one part of the austenitic structure to a structure selected fromthe group consisting of ferrite and martensite and heating the coldworked body at a temperature of about 600 C. to 800 C. for a period oftime sufficient to transform the thus obtained ferritic or martensiticstructure to the sigma phase.

3. A process as claimed in claim 1, wherein said compositionadditionally includes up to 3% of molybdenum, up to 1% of niobium and upto 1% titanium.

4. A process as claimed in claim 1, wherein the cold working isperformed to a reduction of from 10 to and the heating time ranges froma few hours to a few hundred hours.

5. A process as claimed in claim 1, wherein the cold working is oneselected from the group consisting of cold rolling, cold drawing, coldreducing and cold swaging,

'6. A process as claimed in claim 1, wherein after said body having beencold worked below C., said body is cold worked to a reduction of from 10to 95% and is then subjected to heat treatment at a temperature of theorder of 600 to 800 C. for a few hours to a few hundred hours.

7. A process as claimed in claim 1, wherein said body is subjected toshot peening until the surface thereof has a structure selected from thegroup consisting of ferrite and martensite.

References Cited UNITED STATES PATENTS 10 L. DEWAYNE RUTLEDGE, PrimaryExaminer W. W. STALLARD, Assistant Examiner US. Cl. X.R.

